This invention is directed to resin-coated sand compositions and a process for the preparation thereof, and more particularly, to a process for producing resin-coated sand compositions substantially free of nitrogen by the dry hot coating method. The compositions thus prepared are suitable for manufacturing foundry sand cores and molds.
Conventional resin-coated sand compositions have been typically prepared by the dry hot coating method by mixing heated sand particles with a novolac phenolic resin (hereinafter referred to as a novolac resin) in the form of an agitated aqueous solution, adding hexamethylenetetramine (hereinafter referred to as hexamine) as a hardening agent, in an amount of 10 to 15 parts by weight with respect to 100 parts by weight of resin, and further adding calcium stearate.
Such resin-coated sand compositions produced with novolac resins and hexamine exhibit fast curing and excellent flow, but are also often associated with certain drawbacks such as pollution problems and defects in the cast products, such as pinholes or blow holes resulting from the presence of nitrogen compounds, such as ammonia or formaldehyde, generated by the pyrolysis of hexamine when the resin-coated sand compositions are used in iron or steel casting or when forming molds.
In order to avoid such problems, various attempts have been made to employ a shell molding resin with a very low nitrogen content when using dry hot coating methods. A representative attempt in this direction is the use of a solid resole phenolic resin obtained with an ammonia catalyst (hereinafter referred to as a solid ammonia resole resin).
This method is, however, associated with certain drawbacks such as slower hardening as compared to novolac resins cured with hexamine, and unsatisfactory flow (insufficient hot flow) resulting from premature local curing before the sand particles become well-coated in the course of mulling. As a result, the cured articles frequently exhibit significantly lower physical strength than those obtained with a novolac resin and hexamine, and, therefore, this method has not been extensively employed in practice. A method has also been proposed for controlling the degree of reaction of a solid ammonia resole resin in the earlier stages of reaction in order to accelerate the hardening. However, this method still results in insufficient hot flow and thus in insufficient strength of the molded articles.
In addition, a method has been proposed for using a solid ammonia resole resin as a curing agent for novolac resins. This method, though effective for improving the physical strength of the molded article, is associated with the serious drawback of slow curing.
The present inventors have found that these problems can be solved by using, as a curing agent for novolac resins, a lubricant-containing solid resole phenolic resin having a higher content of methylol radicals (hereinafter referred to as a lubricant-containing solid resole resin). The presence of a lubricant in the resin elevates the apparent melting point and thus improves its resistance against blocking.
As this method provides a solid resole resin having a high degree of reactivity (with a higher content of methylol radicals), it is possible to obtain a coated sand composition with a fast curing speed and with a higher crosslinking density. Also, the uniform distribution of lubricant in the solid resole resin assures an improvement in hot flow and uniform mixing with the novolac resin.
Accordingly, it is an object of the present invention to provide a process for producing a resin-coated sand composition having good physical strength and rapid curing by means of the dry hot coating method.
Another object of the present invention is to provide a process for producing a resin-coated sand composition capable of substantially eliminating gas defects in cast products and also capable of preventing the various associated pollution problems.
A still further object of the present invention is to provide a process which utilizes a stable supply of a reactive binder substantially free of blocking characteristics and which provides for the simplified preparation of a more reactive solid resole resin.